netdev - Re: [PATCH net-next] tcp: don't abort splice() after small transfers

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Message-Id: <20121202.202400.1802827154247759864.davem@davemloft.net>
Date:	Sun, 02 Dec 2012 20:24:00 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, w@....eu
Subject: Re: [PATCH net-next] tcp: don't abort splice() after small
 transfers

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sun, 02 Dec 2012 13:49:27 -0800

> From: Willy Tarreau <w@....eu>
> 
> TCP coalescing added a regression in splice(socket->pipe) performance,
> for some workloads because of the way tcp_read_sock() is implemented.
> 
> The reason for this is the break when (offset + 1 != skb->len).
> 
> As we released the socket lock, this condition is possible if TCP stack
> added a fragment to the skb, which can happen with TCP coalescing.
> 
> So let's go back to the beginning of the loop when this happens,
> to give a chance to splice more frags per system call.
> 
> Doing so fixes the issue and makes GRO 10% faster than LRO
> on CPU-bound splice() workloads instead of the opposite.
> 
> Signed-off-by: Willy Tarreau <w@....eu>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied.
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